Method for preparing inorganic porous material

ABSTRACT

Disclosed is a method for producing an inorganic porous body having precisely controlled macropores concurrently with mesopores of a narrow pore size distribution. The method comprises dissolving an amphiphilic substance as template component in an aqueous solution containing a sol-gel reaction catalyst, adding an inorganic low molecular weight compound having hydrolysable functional groups to the solution, forming a gel including a solvent-rich phase for the macropores, drying the gel to remove the solvent, and removing the template component by such means as thermal decomposition.

TECHNICAL FIELD

This invention relates to a novel method for producing an inorganicporous body. The method of the present invention is suitably applied tothe production of porous bodies for use as packing materials forchromatography, porous bodies for blood separation, porous bodies formoisture absorption, deodorizing porous bodies for low molecularsubstance adsorption, porous bodies for enzyme or catalyst supports andthe like.

BACKGROUND ART

As porous materials for use in the above-mentioned applications, therehave been hitherto known those which are composed of organic polymerssuch as styrene-divinylbenzene copolymers, as well as those which arecomposed of inorganic materials such as silica gel. In use, they aregenerally packed in the form of a column.

The organic material columns have disadvantages such as that they havelow mechanical strength against applied pressure, they easily swell orshrink when exposed to solvents, and they cannot be sterilized byheating. Thus, in cases where increased productivities are required byoperation at high temperatures, the general practice is to use inorganicmaterials, particularly silica gel, because they do not suffer from suchdisadvantages.

Inorganic porous materials, such as silica gel are generally produced bythe sol-gel process, which is a liquid phase reaction. As well known,the sol-gel process refers to a general process for producing aggregatesor polymers of an oxide from an inorganic low molecular weight compound,in which the inorganic low molecular weight compound having hydrolysablefunctional groups for use as the starting materials is subjected to asol-gel reaction, i.e., a hydrolysis and subsequently a polymerizationreaction (polycondensation). The best known inorganic low molecularweight compounds for use as the starting materials are metal alkoxides.Other examples include metal chlorides, metal salts or coordinationcompounds having a hydrolysable functional group such as carboxyl groupor β-diketone, and metal amines.

For use as a carrier for various purposes, a porous material should havean optimal median pore size as well as the narrowest pore sizedistribution suitable for the specific substance which is to be carriedon the surface of the porous material for exhibiting the desiredfunction. Thus, for a porous material produced by the sol-gel process,attempts have been made to control the pore size through controlling ofthe reaction conditions for the gel preparation.

Recently, a number of researchers have reported that the preparation ofa porous material by the sol-gel process in the presence of anamphiphilic substance such as a surfactant or a block copolymer (morestrictly, a molecular assembly of such amphiphilic substance formedthrough the self-organization) as a template makes it possible toprecisely control pore structures in the nanometer range. However,conventional porous bodies produced by such sol-gel process generallyhave only nanometer-size pores (i.e., so-called mesopores), in which thebodies are mostly in the form of powder, thin films or irregularparticles. Even if the porous bodies are produced in the form of a bulkmaterial, no examples are found where larger-scale pores (i.e.,so-called macropores) are controllably coexistent.

It is known that a sol-gel reaction for preparing silica gel using abasic catalyst, in the presence of an amide compound or from a siliconalkoxide as the starting material, will result in the enlargement of theaverage pore diameter. However, the resultant material has only pores of20 nanometers, even at the largest, and a pore-size distribution inwhich most of the pores extend to the smaller diameter region.

In use as filters, carriers and other applications, the porous materialshaving only nanometer-sized pores (mesopores) as described above aregenerally pulverized and then packed in a column, in which thepulverized particles may be bonded together. Thus, the substance to betreated (i.e., the gas or liquid as the mobile phase) enters into themesopores, through the spaces formed among the pulverized or bondedparticles, in order to establish a desired function by the porousmaterials. However, many cases are known where the desired function isnot fully exhibited because of insufficient porosity as well as theirregular or nonuniform spaces formed by the porous materials. Acomplicated and time-consuming process is required to produce a porousassembly by which a target substance can be smoothly introduced intonanometer-sized pores (mesopores) for contact therewith, or a bulkmaterial having a macropores structure satisfying such condition.

It is an object of the present invention to provide a new method forproducing inorganic porous bodies having precisely controlled macroporesconcurrently with mesopores of a narrow pore size distribution.

DISCLOSURE OF THE INVENTION

The present inventors have found that the above-mentioned object isachieved by producing an inorganic porous body by a sol-gel process inthe presence of an amphiphilic substance as the template component,under the conditions where the sol-gel transition occurs simultaneouslywith the phase-separation.

Thus, according to the present invention, there is provided a method forproducing an inorganic porous body having macropores concurrently withmesopores, which comprises the steps of

-   -   (i) preparing a homogeneous solution by dissolving an        amphiphilic substance as template component in an aqueous        solution containing a sol-gel reaction catalyst,    -   (ii) adding an inorganic low molecular weight compound having        hydrolyzable functional groups to said homogeneous solution so        as to cause a sol-gel reaction, thereby forming a gel of a        continuous three-dimensional network structure composed of a        solvent-rich phase which is rich in the solvent and a skeleton        phase which is rich in an inorganic oxide polymer adhered to the        surface of the amphiphilic substance template wherein the        inorganic oxide polymer is produced from said inorganic low        molecular weight compound by the sol-gel reaction,    -   (iii) drying said gel to remove the solvent from the        solvent-rich phase by evaporation, thereby forming the        macropores, and    -   (iv) removing said template component from the dried gel by        thermal decomposition or extraction, thereby forming the        mesopores within the skeleton phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning electron microscopic photograph of the structureobtained by the removal of the solvent by evaporation after the sol-gelreaction step in Example 1.

FIG. 2 shows a scanning electron microscopic photograph of the porousbody obtained in Example 1.

FIG. 3 is shows the pore size distribution curve, as measured by themercury porosimetry method and the nitrogen method, of the porous bodyobtained in Example 1.

FIG. 4 shows the pore size distribution curve, as measured by themercury porosimetry method and the nitrogen adsorption method, of theporous body obtained in Example 2 and 3 in which the preparationtemperature was 40° C.

FIG. 5 shows the pore size distribution curve, as measured by themercury porosimetry method and the nitrogen adsorption method, of theporous body obtained in Examples 2 and 3 in which the preparationtemperature was 60° C.

FIG. 6 shows the pore size distribution curve, as measured by themercury porosimetry method and the nitrogen adsorption method, of theporous body obtained in Examples 2 and 3 in which the preparationtemperature was 80° C.

THE BEST MODE FOR CARRYING OUT THE INVENTION

The method of the present invention relates to the production of aninorganic porous body by a sol-gel process in the presence of anamphiphilic substance as the template component, and is characterized inthat it includes the step of forming a gel composed of a solvent-richphase, from which macropores will form in the subsequent step of drying,and a skeleton phase, from which mesopores will form within the phase inthe subsequent step of thermal decomposition or extraction, by adjustingthe reaction conditions so that the sol-gel transition and thephase-separation occur concurrently. By contrast, according toconventional methods for producing porous bodies by a sol-gel process inthe presence of an amphiphilic substance as the template, the resultantporous bodies have only mesopores as mentioned previously. This isprobably because, in the conventional methods, the oxide polymer formstoo quickly and locally on the surface of the template and precipitatesto be separated out of the system.

The terms “macropore” and “mesopore” as used herein are defined inaccordance with the well-known proposal by IUPAC. Thus, a macroporerefers to a pore having a diameter of larger than 50 nanometers while amesopore refers to a pore having a size between macropores and micropores (having a diameter of less than 2 nanometers), i.e., a pore havinga diameter of 2 to 50 nanometers. The porous body produced by thepresent invention generally has mesopores with a narrow pore sizedistribution primarily in the range of 2 to 10 nanometers.

While the principle of the present invention can be applied to a varietyof inorganic low molecular weight compounds from which there are formedoxide polymers by the sol-gel process as described previously inreference to the background art, the present invention is most suitablyapplied to cases where the inorganic oxide polymer for composing theporous body is silica (silica polymer) and/or a polymer of siloxanehaving an organic functional group or groups.

In producing a porous body composed of silica and/or siloxane polymerand concurrently having mesopores and macropores in accordance with thepresent invention, it is necessary to adjust the conditions for sol-gelreaction in such manner that the sol-gel reaction proceeds under anacidic condition at least during the initial stage of the reaction andthat the quantity of the water containing the catalyst is in the rangeof 2.0 to 40.0 g per 1.0 g of silica (in terms of anhydrous silica) inthe reaction system. By such adjustment it is assured that the sol-geltransition and the phase-separation occur concurrently to result in theformation of the gel composed of the solvent-rich phase and the skeletonphase.

More specifically, it is known that, in the preparation of a porous bodymainly composed of silica by a sol-gel reaction using an amphiphilicsubstance as a template, there can be formed uniformly-sized mesoporesdue to the template under any of acidic, neutral or basic catalystcondition. However, for the production of the gel separately composed ofthe solvent-rich phase and the skeleton phase in accordance with thepresent invention, the sol-gel reaction has to be conducted under anacidic condition where uniform hydrolysis and gel formation occureasily.

Uniform hydrolysis and gel formation can also be induced through ahomogeneous reaction taking place inside the reaction solution so thatthe liquid property, which was acidic at the initial stage of thereaction, is gradually converted to basic (for example, by adding ureato the reaction solution so that the urea will be hydrolyzed to produceammonia).

A sol-gel reaction involves the formation of binding sites (the sitesfor polycondensation: typically, hydroxyl groups) through hydrolysis,and the formation of gel through the polycondensation reaction at thebinding sites. It is considered that under an acidic condition thehydrolysis is promoted so as to form a number of the polycondensationsites, through which the polycondensation reaction (the gel formation)proceeds uniformly. By contrast, in a case where a sol-gel reactionproceeds under a basic condition even from the initial stage ofreaction, the polycondensation is promoted rather than the hydrolysis,which will induce a nonuniform gel formation. Catalysts for use in thesol-gel reaction include, but are not limited to, a mineral acid such ashydrochloric acid, nitric acid or sulfuric acid, an organic acid such asacetic acid or citric acid, a weak base such ammonia or amines, or astrong base such as sodium hydroxide or potassium hydroxide, in whichcontrolling the liquid property for the reaction is more important thanthe type of the catalyst.

Water content in the sol-gel reaction is another important factor inproducing a porous body composed of silica or siloxane polymer andconcurrently having mesopores and macropores in accordance with thepresent invention. The quantity of water containing the catalyst shouldbe in the range of 2.0 to 40.0 g, preferably 3.0 to 20.0 g, morepreferably 4.0 to 10.0 g per 0.0167 moles of silicon atom (i.e., 1.0 gcalculated in terms of anhydrous silica). An excess amount of water willresult in the precipitation of polymers having an insufficient degree ofpolymerization into the water without uniform gel formation. This is incontrast to the fact that, in the conventional method of producingporous bodies having only mesopores, by means of a sol-gel process inthe presence of an amphiphilic substance as the template, the watercontent as defined in the above is generally more than 50 g, and in somecases, more than 100 g.

Thus, according to the present invention, a sol-gel reaction process iscontrolled in such manner that the sol-gel transition and the phaseseparation occur substantially concurrently, thereby forming a gelcomposed of a solvent-rich phase which is rich in the solvent (water)and a skeleton phase which is rich in the oxide polymer, wherein suchformation is ascertained by the fact that the solution becomes turbidwithout giving rise to any precipitation. The gel product furthersolidifies when aged for some time (while being slightly warmed ifnecessary). The resultant gel is then subjected to drying and thermaldecomposition (or extraction) to produce a desired porous body.

The method of the present invention for producing an inorganic porousbody concurrently having mesopores and macropores thus starts with thestep of preparing a homogeneous solution by dissolving an amphiphilicsubstance as template component into an aqueous solution containing asol-gel reaction catalyst. When an inorganic low molecular weightcompound having hydrolysable functional groups is added to thehomogeneous solution so as to cause a sol-gel reaction, there is formeda gel composed of separate phases of a solvent-rich phase and a skeletonphase as described in the preceding.

The solvent-rich phase is a continuous three-dimensional network phasehaving a width corresponding to a macropore diameter. This can beascertained by the microscopic observation of the structure producedafter removing the solvent by drying, as will be described later (cf.FIG. 1).

The skeleton phase is rich in the inorganic oxide polymer produced fromthe inorganic low molecular weight compound by the sol-gel reaction andis also a continuous three-dimensional network phase. This phase is madeof the inorganic oxide polymer adhered to the surface of the substanceas the template component (more strictly, a molecular assembly of theamphiphilic substance formed by self-organization), as is evidenced bythe formation of small pores (mesopores) within the skeleton phase bythe subsequent removal of the template component, the amphiphilicsubstance (cf. FIG. 2). Thus, the oxide polymer has on its surfacehydroxyl groups, which strongly interact with the proton-accepting siteson the amphiphilic substance, thereby enabling the structure of thetemplate formed in the solution due to the self-organization to betransferred into the gel network.

As the product of the sol-gel reaction (the sol) solidifies, followed byaging for an appropriate time, and the solvent is removed by drying, thevolume occupied by the solvent-rich phase becomes interconnectedmacropores. Then the removal of the template component, the amphiphilicsubstance, by thermal decomposition or extraction will produceuniformly-sized pores in the nanometer range (mesopores) formed from theself-organized structure of the template component.

The amphiphilic substance for use in the present invention as templateis preferably a surface-active agent of cationic surface-active agent ornonionic surface-active agent having a hydrophilic portion such asquaternary ammonium salt as well as a hydrophobic portion typically ofan alkyl group, or a block copolymer having a hydrophilic portion and ahydrophobic portion. Concrete examples include, but are not limited to,halogenated alkylammonium, polyoxyethylene alkylether, and ethyleneoxide-propylene oxide-ethylene oxide block copolymer. The amphiphilicsubstance for use in the present invention is preferably one which canbe homogeneously dissolved in the reaction solution, such as asurface-active agent or the above-exemplified block copolymer.Furthermore, as can be seen from the foregoing description, theamphiphilic substance used in the present invention not only functions,as a template, to define the diameter of nanometer-sized pores(mesopores) but also functions as a co-existent substance in theformation of the solvent-rich phase from which the macropores areproduced. The amount of the template component is preferably in therange of 0.5 to 5.0 g, more preferably 1.0 to 3.0 g, the most preferably1.5 to 2.5 g per 0.0167 moles of silicon atom (1.0 g in terms ofanhydrous silica).

As inorganic low molecular weight compounds having hydrolysablefunctional groups, there can be used a variety of metal compoundsincluding metal alkoxides as described in the preceding with respect tothe background art. In producing porous bodies composed of silica inaccordance with the preferred embodiment of the present invention,monomer or low molecular weight polymer (oligomer) of a silicon alkoxideis suitably used as the silica source. In the production of porousbodies composed of a siloxane polymer having an organic functional groupor groups (organic-inorganic hybrids), there can be used, as the sourceof such organic-inorganic hybrid, monomer or low molecular weightpolymer of silicon alkoxide having at least one silicon-carbon bond, ora compound with a structure in which two or more silicon atoms arebridged via one or more carbon atoms (e.g., bis(trialkoxysilil)alkanes). Inorganic porous bodies composed of silica in combination witha siloxane polymer having an organic functional group or groups can alsobe produced in accordance with the present invention.

EXAMPLES

The characteristic features of the present invention will be furtherclarified with reference to the following examples, but should not beconstrued to be limited thereto.

Example 1

Cetyltrimethylammonium bromide, a cationic surface-active agent (alkylchain carbon number: 16, available from Tokyo-kasei) 1.0 g was dissolvedin 1 mol/L nitric acid aqueous solution 9.0 g. To the resultanthomogeneous solution was added tetramethoxysilane 5.15 g while stirringto carry out hydrolysis. The amount of water containing the catalyst was4.43 g per 1.0 g of silica. After stirring for several minutes, theresulting clear solution was transferred into a closed vessel, which wasthermostatted at 60° C. In some 120 minutes, the solution became turbidand then solidified.

The solidified sample was further aged for several hours, followed byremoval of the solvent by evaporation at 60° C. FIG. 1 is a scanningelectron microscopic image of the structure thus produced. As can beseen from the figure, there was formed a three-dimensional networkstructure composed of macropores (the black areas in the figure) and askeleton phase (the white areas in the figure).

The dried gel was then heated up to 650° C. at the rate of 100° C./hr.Thus, there was produced a porous body composed of amorphous silica. Theporous body had a three-dimensional network structure of uniformly-sizedthrough pores (macropores) with a median diameter of 3 μm (=3000 nm) anda gel skeleton (a skeleton phase) with a width of about 2 μm, asevidenced by microscopic observation (FIG. 2) and mercury porosimetrymeasurement (FIG. 3). It was also ascertained by nitrogen adsorptionmeasurement that on the inner surface of the through pores (within theskeleton phase) there were a number of small pores having a diameter of3 nm or smaller, providing a specific surface area of larger than 400m²/g.

Example 2

Ethylene oxide-propylene oxide-ethylene oxide block copolymer(EO20-PO70-EO20, average molecular weight: 5800, Aldrich), anamphiphilic substance, 2.10 g was dissolved in 1 mol/L nitric acidaqueous solution 10.0 g. To the resultant homogeneous solution was addedtetramethoxysilane 5.15 g while stirring to carry out hydrolysis. Theamount of water containing the catalyst was 4.93 g per 1 g of silica.After stirring for several minutes, the resultant clear solution wastransferred into a closed vessel, which was thermostatted at 40° C. Insome 120 minutes, the solution became turbid and solidified.

The solidified sample was further aged for several hours, followed byremoval of the solvent by evaporation at 60° C., and then heating up to650° C. at the rate of 100° C./hr. Thus, there was obtained a porousbody composed of amorphous silica.

The resultant porous body had a three-dimensional network structurecomposed of uniformly-sized through pores with a median diameter ofapprox 3 μm (=3000 nm) and a gel skeleton with a width of approx 2 μm,as evidenced by microscopic observation and mercury porosimetrymeasurement. It was ascertained by nitrogen adsorption measurement thaton the surface of the through pores there were a number of pores with adiameter distribution of mainly approx 3 nm, providing a specificsurface area of larger than 400 m²/g. The pore size distribution isshown in FIG. 4.

Example 3

Ethylene oxide-propylene oxide-ethylene oxide block copolymer(EO20-PO70-EO20, average molecular weight: 5800, Aldrich), anamphiphilic substance, 2.10 g was dissolved in 1 mol/L nitric acidaqueous solution 10.0 g. To the resultant homogeneous solution was addedtetramethoxysilane 5.15 g while stirring to carry out hydrolysis. Afterstirring for several minutes, the resultant clear solution wastransferred into a closed vessel, which was thermostatted at 60° C. or80° C., and in some 60 minutes or 40 minutes, respectively, thesolutions became turbid and solidified.

The resultant porous bodies had a three-dimensional network structurecomposed of uniformly-sized through pores with a median diameter ofapprox 1 μm (=1000 nm) and a gel skeleton with a width of approx 1 μm,in the case where the sample was treated at 60° C., and athree-dimensional network structure composed of uniformly-sized throughpores with a median diameter of approx 0.2 μm (=200 nm) and gel skeletonwith a width of approx 0.2 μm, respectively, as evidenced by microscopicobservation and mercury porosimetry measurement. The pore distributionsare shown in FIG. 5 and FIG. 6, respectively.

INDUSTRIAL UTILITY

As described, according to the present invention it is possible toproduce porous bodies having a pore distribution controlled as desired.The porous bodies produced by the present invention have a dual porestructure composed of macropores and mesopores, and hence are applicablenot only as materials for packed-type column chromatography in which theparticulate porous bodies are packed in a cylinder but also as materialsfor integral-type column chromatography in which the porous bodies prese constitute a column.

1. A method for producing an inorganic porous body having macroporesconcurrently with mesopores, which comprises the steps of (i) preparinga homogeneous solution by dissolving an amphiphilic substance astemplate component in an aqueous solution containing a sol-gel reactioncatalyst, (ii) adding an inorganic low molecular weight compound havinghydrolyzable functional groups to said homogeneous solution so as tocause a sol-gel reaction, thereby forming a gel of a continuousthree-dimensional network structure composed of a solvent-rich phasewhich is rich in the solvent and a skeleton phase which is rich in aninorganic oxide polymer adhered to the surface of the amphiphilicsubstance wherein the inorganic oxide polymer is produced from saidinorganic low molecular weight compound by the sol-gel reaction, (iii)drying said gel to remove the solvent from the solvent-rich phase byevaporation, thereby forming the macropores, and (iv) removing saidtemplate component from the dried gel by thermal decomposition orextraction, thereby forming the mesopores within the skeleton phase. 2.The method for producing an inorganic porous body as claimed in claim 1,wherein the inorganic oxide polymer is silica and/or a polymer ofsiloxane having an organic functional group or groups.
 3. The method forproducing an inorganic porous body as claimed in claim 2, wherein thesol-gel reaction is carried out under an acidic condition at leastduring the initial stage of the reaction, and the quantity of watercontaining the catalyst is in the range of 2.0 to 40.0 g per 1.0 g ofsilica (in terms of anhydrous silica) in the reaction system.